Holding Tank With Internally Reinforced Sidewalls and Liquid Dispenser Using Same

ABSTRACT

The inward and outward deflection of sidewalls of a holding tank for liquids is reduced or eliminated by sidewall reinforcing stringers that extend between the sides and which are located inside the tank. The stringers, maintain the separation distance between the sidewalls to keep the sidewalls substantially parallel to each other.

BACKGROUND

This application relates to liquid holding tanks having one or more relatively thin, planar side walls. More particularly, this invention relates to liquid holding tanks made of thin and flexible materials, the shapes of such tanks being reminiscent of rectangular parallelepipeds.

As used herein, a parallelepiped is considered to be a volume or body bounded by four rectangles and two parallelograms. A rectangular parallelepiped, however, is a parallelepiped, all six faces of which are rectangles or substantially rectangular. A rectangular parallelepiped is also considered to be a right prism, the bases of which (top and bottom surfaces) are parallelograms. A cube is a rectangular parallelepiped because it is a volume or body bounded by six identical squares, the squares being rectangles.

A problem with holding tanks having thin, flat side walls is that such walls tend to bow outwardly when the tank contains liquid because of the distributed force exerted on the walls by the liquid. As the level of the liquid in the tanks rises, the outward deflection of the walls increases. Outward deflection of sidewalls used in a rectangular, parallelepiped-shaped tank can make it difficult to remove the tank from a space wherein the nominal tank width is such that it just fits into the space.

The inward deflection of a tank's sidewalls can also be problematic. In many liquid holding tanks, the level of the upper surface of the liquid held in the tank is used to determine the volume held in the tank. The height of the upper level also determines the pressure at the bottom of the tank. Since most liquid dispensers draw liquid from the bottom of the tank, the height of the upper level will determine not only the flow rate from the tank but also the apparent volume left in a tank. A liquid holding tank with thin walls that are able to be held upright would be an improvement over the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of three liquid holding tanks;

FIG. 2 is a perspective view of the left-hand tank shown in FIG. 1;

FIG. 3 is a cutaway view of the tank shown in FIG. 2 taken along section lines 3-3;

FIG. 4 is a perspective view of the center tank shown in FIG. 1;

FIG. 5 is a cutaway view of the tank shown in FIG. 4 taken along section lines 5-5;

FIG. 6 is a perspective view of the right-hand tank shown in FIG. 1;

FIG. 7 is a cutaway view of the tank shown in FIG. 6 taken along section lines 7-7;

FIG. 8 is a front elevation view of a liquid dispenser having holding tanks with reinforced sidewalls;

FIG. 9A is an isolated perspective view of a right-hand portion of an alternate embodiment of a stringer;

FIG. 9B is a sectional view of a stringer shown in FIG. 9A; and

FIG. 10 is a sectional view of the right-hand tank shown in FIG. 6 with an alternate embodiment of a stringer.

DETAILED DESCRIPTION

FIG. 1 is a front view of an assembly of liquid holding tanks 100. The assembly 100 is comprised of a left-hand tank 102, a center tank 104, and a right-hand tank 106. Each tank has a top 108 and a bottom 110.

The left tank 102 and the right tank 106 both have a nominal width W₁, which is less than the nominal width W₂ of the center tank 104. All three tanks have the same nominal height, H. All three tanks are the same nominal depth, which is considered to extend into the plane of the page on which FIG. 1 is drawn.

The tanks' various dimensions and their resultant sizes are design choices. The tanks' sizes shown in FIG. 1 are for illustration purposes only.

All three tanks are considered to have “thin” sidewalls. The side exterior surfaces are also smooth.

The term “thin” should be considered to mean between about 0.4 millimeters (0.015 inches) and about 4 millimeters (0.157 inches). The term “smooth” is considered herein to mean that the exterior sidewall surfaces are without a projection, which if between two tanks would prevent the two adjacent sides from making contact with each other.

Although the sidewalls are thin and thus susceptible to outward deflection or bowing when the tanks hold liquid and susceptible to inward deflection or bowing because of manufacturing defects or an object pressing inwardly, the side walls of the tanks are reinforced to reduce or eliminate outward deflection when the tanks hold liquid, and reduce or eliminate inward deflection or deformation. The tanks shown in FIG. 1 are considered to be “sandwiched” against each other because the sides of the tanks are in contact with each other. They are nevertheless able to freely slide against each other, including freely moving vertically relative to each other. The left side outside sidewall of the left tank 102 and the right side outside sidewall of the right tank 106 contacts adjacent surfaces of a refrigerated cabinet, not shown in FIG. 1.

The physical contact between the smooth surfaces of the tanks' sidewalls provides an enhanced heat transfer path between the tanks, liquids contained inside them and the sidewalls of a heated or cooled cabinet, when the tanks and a cabinet for them are sized, shaped and arranged to provide an intimate contact between them, as shown in the Applicant's co-pending patent applications. One such application is application Ser. No. 12/885,659 filed on Sep. 20, 2010, and which is entitled, “Dispenser for Liquids,” the entire contents of which are incorporated by reference. Another application is application Ser. No. 13/169,339, filed Jun. 27, 2011 and which is entitled “Liquid Dispenser with Storage Tanks” The entire contents of that patent application (application Ser. No. 13/169,339) are also incorporated herein by reference.

The tank assembly 100 is configured for use in a liquid dispenser, by which is meant that the tanks are sized, shaped and arranged to be used in a liquid dispenser. Examples of such dispensers can be found in the aforementioned patent applications incorporated herein by reference. The sidewalls are preferably thin in order to reduce the tanks' cost but to also reduce their weight while maximizing the volume of liquid they can hold in a relatively small refrigerated cabinet.

FIG. 2 is a perspective view of the left-hand tank 102. The left-hand tank 102 has a substantially planar front face or side 200, a substantially planar rear side or face 202 not visible in FIG. 2, a substantially planar left side 204, not visible, and a substantially planar right side 206. All four sides are substantially planar, i.e., at least their exterior surfaces are flat as well as smooth, and joined to each other by stress-relieving, curved corner sections 208.

The tanks 102, 104 and 106 have substantially equal-sized tops 108. The equal-sized tops 108 makes a tank re-filling task equally convenient (or inconvenient), for each tank. Providing each tank with an equal-sized top 108, however, requires the right-side of the left-hand tank 102 to curve rightward near the tank's top. It also requires the left side of the right-hand tank 106 to curve leftward near the top.

Each tank shown in FIG. 1, including the left-hand tank shown in FIG. 2 and FIG. 3 has a cover 210 that can also be considered to be a top for a tank. The cover 210 is provided with a hinged door 212, which allows the tank 102 to be refilled. The door 212 is attached to the cover 210 by a hinge and opens by rotation around a hinge pin 214 “rotatably” attached to the cover 210.

The term, “rotatably attached” means that the door 212 is attached to the cover 210 so that the door 212 can rotate along one edge of the cover, about an axis, yet be effectively attached to the cover 210. A hinge provides a rotatable attachment.

The front side or face 200, rear side or face 202, the left side 204, and the right side 206 are considered herein to be walls. As can be seen in the figure, the front wall 200 and the rear wall 202 are substantially parallel to each other as are the left wall 204 and the right wall 206. Minor deviations in the parallelism of the sides are likely to exist at least because of manufacturing tolerances.

Problematic outward deflection and problematic inward deflection is eliminated or at least significantly reduced in thin-walled liquid holding tanks, such as the tanks shown in FIG. 1, by providing sidewalls that oppose each other with sidewall reinforcements or stiffeners. The sidewall stiffeners described herein extend between the walls, but are located inside the exterior surfaces of the sidewalls. They do not add or require a projecting fastener outside the tank's sidewalls. The sidewall stiffeners, which are referred to hereinafter as stringers, are in tension when the tank holds liquid. The stringers help to hold the walls rigid and upright, at least where they are attached to the sidewalls, and thus maintain wall separation distance, at least where they are attached to the sidewalls, when the tank holds liquid.

Except for the cover 210, all four sidewalls, the stringers and the bottom 110 are formed by rotational molding or “roto-molding.” Rotational molding is also known as spin casting and “rotocasting.” It is a process wherein a thin-walled, two-piece mold is designed to be rotated about two perpendicular axes. Rotational molding is able to produce hollow parts, including the tanks 102, 104, 106 with side wall thicknesses as small as 0.4 millimeters. See, Serope Kalpakjian, et al., Manufacturing Engineering and Technology, 501-502, Pearson Education, Inc. (2010).

To mold the tanks using rotational molding, a premeasured quantity of powered plastic material is placed inside a tank form or mold. The mold is heated as it is rotated around the two perpendicular axes. Rotation of the mold around the two axes tumbles the plastic power against the mold. Depending on the type of plastic, the heat can either fuse the powder or it can melt and coat the mold surfaces. The material in the mold eventually solidifies whereupon the mold is opened and the molded piece removed from the mold.

By forming the tank using rotational molding, the stringers are formed to extend between the sidewalls and are formed seamlessly, i.e., without seams. As used herein, a seam exists where two pieces are joined together and should not be confused with a mold mark And, since the material from which the tank and its stringers are formed is continuous or seamless, a stringer is formed integrally with the sidewalls during the spin molding process. A stringer is thus considered herein to be part of, or formed with the sidewalls. The rotational molding process also enables the stringers to be formed inside the tank without a projection or protuberance above the outside surfaces of the sidewalls as a thru-bolt might require. The thickness of the sidewalls and the stringers formed with them are also the same or substantially the same. The stringers do not require any sort of localized sidewall mounting boss or reinforcement that a thru-bolt recessed into a sidewall might require.

In FIG. 2, the left wall 204 and the right wall 206 are reinforced and helped to stay vertical by at least one stringer that extends between the sidewalls 204 and 206. FIG. 2 shows two stringers 220-1 and 220-2. They are formed during the same spin casting process during which the sidewalls 204 and 206 are formed. The stringers are thus part of the sidewalls 204 and 206 that oppose each other. The stringers 220-1 and 220-2 are hollow and because they are formed during the spin casting process, they are as thin as the sidewalls but nevertheless significantly reduce outward and inward deflection of the sidewalls 204 and 206.

FIG. 3 is a cross sectional view of the tank 102 shown in FIG. 2 taken through section lines 3-3. FIG. 3 depicts the left-side tank shown in FIG. 1. The outside surfaces 300 of the sidewalls 204 and 206 are depicted as being upright or at least substantially upright, vertical or at least substantially vertical, and planar or at least substantially planar, i.e., without a projection such as a thru-bolt 301 that would be required if the sidewalls were to be reinforced or stiffened by such a structure, and smooth.

The stringers 220-1 and 220-1 are essentially hollow elongated cylinders or tubes. They can be seen to be “similar” to each other. Except for their locations in the side walls 204 and 206, the second stringer 220-2 has the same or substantially the same attributes of the first stringer 220-1.

While the stringers 220-1 and 220-2 are essentially tubes, which have themselves a sidewall, both stringers have cross-sectional shapes reminiscent of a venturi, when viewed along section lines 3-3. The stringers have a relatively large inside diameter at the tank sidewalls 204, 206. The stringers' diameter narrows continuously throughout the length of a tapered region 304 of the stringers. The inside and outside diameters of the stringers narrows to a constriction, also referred to as an inflection point 306 where the stringer's inside diameter is most narrow. The relatively large taper of the stringers that imbues them with a venturi-like cross sectional shape is attributable to the draft used in the mold from which the tank is made. The larger the draft used in a mold the easier it is to remove a molded part. The draft of the stringer is preferably between about two degrees and twenty degrees to enable the molded tank 102 to be easily removed from the mold in which it is formed.

As used herein, the terms “draft,” “draft angle” and “taper” refer to an angle of a surface relative to a reference, typically vertical or horizontal, which is provided to a mold to enable of a part from the mold after the part has solidified. The ease with which a molded object can be removed from a mold depends in part on the draft angle used in the mold. Draft angles can be small for molded metals, e.g., slightly greater than one (1) degree, however, draft angle are usually larger for molded plastics. The draft angle us usually measured with respect to a vertical line or axis but can be measured relative to a horizontal line or other reference line.

Those of ordinary skill in the art will recognize that the tanks can be made from separately-molded tank halves that are mirror “images” of each other or substantially mirror images of each other. Tanks formed from separate halves are joined together after being molded by heating or an adhesive.

A hollow stringer 220 reduces the weight and thus the cost of the tanks but an additional benefit of a hollow stringer is that air can flow through the open space inside the hollow stringer. Air inside a stringer can enhance heat transfer into or out of a liquid inside the tank. By way of example, air inside a stringer 220 that is cooler than the stringer material 309, or that is cooler than liquid inside the tank will absorb heat. Air in a stringer 220 that is warmer than the stringer material 309 or warmer than liquid in the tank will transfer heat into the stringer and thus into the liquid in the tank. A stringer is thus considered to also be a heat exchanger.

Despite the fact that the stringers 220-1 and 220-2 are integrally formed with the sides, are continuous and seamless, for purposes of claim construction, the stringers 220-1 and 220-2 are considered herein to have opposing first and second ends, 310 and 312 respectively. The lower or first stringer 220-1 has a first end 310 located at but also forming part of the left-hand wall 204; a second end 312 is located at but also forming part of the right-hand wall 206. Similarly, the upper or second string 220-2 has a first end 310 located at and forming part of the left-hand wall 204 and a second end 312 located at and forming part of the right-hand wall 206. The stringers are preferably formed to be substantially orthogonally, i.e., at a right angle to the sidewalls that they reinforce.

In FIG. 2 and FIG. 3 the stringers 220-1 and 220-2 are vertically separated from each other. The distance between the stringers is considered to be a stringer separation distance 312. The stringer separation distance is selected to keep the center line 314 of each stringer 220 equidistant or substantially equidistant in a vertical direction, i.e., above or below, a geometric point or location on the sidewalls 204 and 206, which is considered herein to be a “centroid” for each side 204 and 206.

The geometric center of the sides, i.e., the centroid in each wall, is identified by reference numeral 230. The stringer separation distance 312 is considered herein to be the distance between a geometric axis or center line 314 of each of the two stringers. In one embodiment of a holding tank with internally-reinforced sidewalls, the wall stiffener is located between the sidewalls at the centroids of each wall. In preferred embodiments, wherein multiple stiffeners are used, the stiffeners can be spatially separated from each other and located in the sidewalls to be equidistant or substantially equidistant from the centroid 230 in order to equalize the retention force provided by each stringer. In an alternate embodiment, two or more stringers are horizontally separated from each other in the sidewalls to be at the same level or distance from the bottom 110 of the tank but on either side of the centroid 230. In yet another embodiment having four stringers, two of the four stringers being located as shown in FIG. 3, two other stringers being laterally separated from each other on either side of the centroid 230 and at the same distance from the bottom 110.

FIG. 4 is a perspective view of the center tank 104 shown in FIG. 1. As with the tank shown in FIG. 3, the walls are upright, substantially planar and have exterior surfaces that are substantially smooth. The center tank 104 has a front wall 400 and an opposing rear wall 402. Similarly it has a left-hand wall 404 and an opposing right-hand wall 406. The four walls, 400, 402, 404 and 406 are substantially planar and joined to each other by stress-relieving, curved corner sections 408. The opposing left and right sidewalls 404 and 406 are held substantially upright with smooth exterior surfaces 410. Their outward and inward deflection is reduced or eliminated by two stringers 440-1 and 440-2, both of which are hollow to allow air to flow in and through them, both of which extend between the two opposing sidewalls 404 and 406. As with the stringers 220-1 and 220-2 depicted in FIGS. 2 and 3, the stringers 440-1 and 440-2 are also formed together with the sidewalls and are thus continuous and seamless with the sidewalls, i.e., the stringers are formed integrally with the rest of the tank.

The center tank 104 is shown in cross section in FIG. 5, which is a cross section through section lines 5-5. The center tank 104 is also formed by rotational molding. As with the stringers used in the left-hand tank, the stringers 440 used in the center tank are preferably located in the sidewalls so that they are vertically above and below and equally separated from what is considered herein to be a centroid or geometric center 430 for the left side wall 404 and the right-hand wall 406. Alternate embodiments include a tank with laterally or horizontally separated stringers located on either sides of the centroid 430.

As used herein, the centroid is considered to be a geometric center of the side walls. The actual location of the centroid will vary depending upon the geometry of the sidewalls.

For purposes of claim construction, the walls of each tank are thin but nevertheless have two opposing surfaces referred to as “interior” and “exterior” surfaces. As shown in FIG. 5, which is a cross-sectional view of the middle tank 104 shown in FIG. 4, a first interior surface 450 faces into the tank; a second exterior surface 452 faces outwardly. Both surfaces are preferably smooth.

The right-side wall 406 is also thin and it too has opposing surfaces 454 and 456. A first, interior surface 454 of the right-side wall 406 faces into the tank; a second opposing exterior surface 456 is smooth and faces outwardly.

A first portion 458 of a wall stiffener or “stringer” 440 extends into the tank 104 from the left-side wall 404. The first portion 458 is tapered and has a length 460 that extends into the interior 462 of the tank 104. A second portion 464 of the stiffener 440, which is also tapered, extends inwardly from the interior surface 454 of the right side wall 406. The second portion 464 has its own length 466, which is preferably equal to the first length 460. The length 460 of the first portion 458 is measured from the vertical, internal center line 470 of the tank 104 to the internal surface 450 of the left side 404. The length 466 of the second portion 464 is measured from the internal center line 470 to the internal surface 454 of the right side 406. While the lengths 460 and 466 are depicted in FIG. 5 as being equal or at least substantially equal, the lengths of the portions of the stringers can in fact be different from each other. As with the stringers 220-1 and 220-2, air can flow into and through the stiffeners 440-1 and 440-2 shown in FIGS. 4 and 5.

FIG. 6 is a perspective view of the right-hand tank 106 shown in FIG. 1. As with the other two tanks, the right-hand tank 106 has four walls, 600, 602, 604 and 606, joined to each other by stress-relieving curved sections 608. The front wall 600 and the opposing rear wall 602 are substantially planar, and parallel to each other. The left wall 604 and the right wall 606 are planar, have exterior surfaces 610 that are smooth and which are substantially parallel to each other. Two stringers 660-1 and 660-2 that extend inwardly from the sidewalls resist outward and inward deflection of the sidewalls when the tank is filled or partially filled with liquid, not shown.

Reference numeral 630 identifies the location of a centroid or geometric center of the sidewalls 604 and 606. The stringers 660-1 and 660-2 extend inwardly from the left-hand wall 604 and inwardly from the right-hand wall 606, as they do with the other two tanks described above.

The right-hand tank 106 is shown in cross section in FIG. 7. As with other tanks described above, the tank 106 shown in FIGS. 6 and 6 and the included stringers 660-1 and 660-2 are formed by rotational molding. The stringers 660-1 and 660-2 are preferably located in the sidewalls 604 and 606 so that they are above and below and equally separated from a centroid 630 for the left side wall 604 and the right-hand wall 606, in order to evenly distribute the sidewall retaining forces they provide to the sidewalls.

For purposes of claim construction, two more sidewall-reinforced tanks, such as the three sidewall-reinforced tanks 102, 104 and 106 shown in FIG. 1, are considered herein to be an assembly of reinforced liquid holding tanks. Each tank has at least two opposing sidewalls, the exterior surfaces of which are preferably smooth, nominally parallel to each other and separated from each other by a separation distance substantially equal to the width, w, of the tank. The sidewalls of a tank are kept substantially parallel to each other by at least one wall stiffener that extends inwardly from, and between the two opposing sidewalls. The stiffener is a structure that is formed as part of the opposing sidewalls or which is attached to or joined to the sidewalls such that the stiffener does not require a protuberance above the exterior surface of a tank sidewall providing the exterior surface of the sidewall with a smooth surface. The fact that the sidewalls are smooth and kept vertical or substantially vertical reduces and can even eliminate friction between adjacent tanks in a cabinet. That the sidewalls are smooth can also enhance heat transfer between tanks, the sidewalls of which are in contact with each other or with the interior sidewalls of a temperature-controlled cabinet such as a refrigerator.

A liquid dispenser with liquid holding tanks having reinforced sidewalls as described above, the tanks of which are able to move vertically with respect to each other even while they contain liquid, can be realized by combining the tanks described above with a refrigerated liquid dispenser as described in the Applicant's co-pending patent application having U.S. patent application Ser. No. 13/169,339, filed Jun. 27, 2011 and which is entitled “Liquid Dispenser with Storage Tanks” The entire contents of that patent application (application Ser. No. 13/169,339) are therefore incorporated herein by reference. When two or more reinforced tanks as described above are used in a liquid dispenser, such as the one described in U.S. patent application Ser. No. 13/169,339, one advantage is that the friction between the sides of adjacent tanks is reduced because the sides of the tanks are kept upright. An additional advantage is realized by the stringers' facilitating heat transfer between air inside the hollow stringers of the different tanks that are aligned with each other and temperature-controlled sidewalls of a cabinet. Aligned hollow stringers form a heat exchanger, which is effectively located inside the tank and effectively immersed in liquid in the tank.

In FIG. 8, reference numeral 800 identifies the liquid dispenser identified by reference numeral 100 in the aforementioned co-pending application Ser. No. 13/169,339. A left-hand tank 802, a center tank 804 and a right-hand tank 806 are provided with hollow stringers, 808, 810 and 812 respectively and which are aligned to each other vertically and horizontally (into and out of the plane of the figure) such that air can flow between them. Heat energy in the air that is inside the stringers can be absorbed into sidewalls 814 and 816 when such sidewalls are refrigerated. Conversely, heat energy in heated sidewalls 814 and 816 can be transmitted into the air inside the stringers from which it will flow into the tanks. The stringers 808, 810 and 812 act as heat exchangers with respect to air and liquid in the tanks 802, 804 and 806. In FIG. 8, heat energy is represented by small arrows drawn to show heat being dissipated into air inside the stringers 808, 810 and 812. Heat energy is also depicted as flowing through sidewalls of the tanks into the sidewalls 814 and 816 of the dispenser 800.

While the preferred embodiment of a tank and stiffener is formed by rotational molding, in an alternate embodiment, a tank can be formed from joined-together molded halves. In such a tank, each of the halves has a stiffener embodied as cone-shaped, inwardly-directed projection, such as the cone-shaped projection 900 depicted in FIG. 9 as extending inwardly from a tank sidewall 902. The projection 900 is tapered 904 with a flange 906 at the distal end 908. The flange 906 provides a surface that can be joined by heat or adhesive to a mating surface on a similar projection from the opposite side of the tank. Alternate and equivalent embodiments include non-cone shaped inwardly-directed projecting portions. Alternate and equivalent embodiments also include inwardly-directed projections that are of unequal lengths.

FIG. 10 depicts a cross sectional view of yet another embodiment of a thin-walled rectangular liquid holding tank 1000. The particular tank 1000 shown in FIG. 10 is shaped to be used on the right-hand side of the tank assembly shown in FIG. 1. A top portion 1002 of the left-hand side 1004 of the tank 1000 has an elbow or curve 1006 that conforms to the shape of the right-hand side of the center tank 104.

Unlike the tanks described above, the tank 1000 shown in FIG. 10 is molded without a stiffener or stringer. Stated another way, the stringers in the tank shown in FIG. 10 are not formed as part of the tank side walls during a molding process. Solid or semi-solid, cylindrical wall stiffeners 1008 and 1012 are instead added to the interior of the tank 1000 after the tank 1000 is molded. The after-molding stringers are preferably added to the tank by being placed therein through the top 1003 of the tank, which can be removed. One or more sidewall stiffeners are then placed inside the tank and attached to the sidewalls.

In FIG. 10, a top stiffener 1008 has a length L that fits just inside the side walls 604 and 606. The top stiffener 1008 is attached to the sidewalls 604 and 606 using a common mechanical fastener such as a screw 1010.

A bottom stiffener 1010 having the same length as the top stiffener 1008 is attached to the side walls 604 and 606 by either by a localized heating of the stiffener 1010 and sidewall, an ultrasonic weld 1014 between the stiffener and sidewall or an adhesive 1016 or both welding and an adhesive or heat and an adhesive.

Whether the tanks are formed by rotational molding or by assembling the tanks in halves or by other methods, the wall stiffeners described above are considered herein to be tubes. A tube need not have a circular cross section. A tube that forms a wall stiffener can have non-circular cross-sectional shapes, the tube cross-sectional shape being the cross-sectional shape orthogonal to the center line or axis 314, as shown in FIG. 3.

The wall stiffeners referred to above as a stringer, provides an internally-located reinforcement to sidewalls of a holding tank for liquids. The stringers in the embodiments depicted in FIGS. 1-9 are considered to be connected or attached to the sidewalls, even though the stringers formed by rotational molding are integrally formed as part of the sidewalls during a molding process.

The stringers are considered herein as “extending” between the walls. They maintains a tank width by opposing outward-directed force caused by liquid inside the tank and inward deflection that can be caused by manufacturing differences or outward forces applied to the tank sidewalls. When the assembly of tanks is disposed inside of a cabinet, such as a refrigerated dispensing cabinet described in the co-pending patent application identified above, or as shown in FIG. 8, the reinforced sidewalls of the liquid holding tanks allow the tanks to be individually refilled or emptied without having them deform in response to liquid inside one or more of them. The reinforcement also helps keep the sidewalls rigid and as shown in the figures, upright, so that measurement of the volume inside the tanks determined by the level of the liquid can be made more accurately.

The foregoing description is for purposes of illustration only. The true scope of the invention is set forth in the following claims. 

What is claimed is:
 1. A liquid holding tank comprising: a first wall; a second wall opposite the first wall, the first wall and the second wall being thin; and a first stringer inside the liquid holding tank and extending between the first and second walls, the stringer configured to reduce deflection of the first and second holding tank walls.
 2. The liquid holding tank of claim 1, wherein the first and second walls are substantially planar, substantially vertical, substantially parallel to each other and wherein the stringer is formed integrally with both the first wall and second wall.
 3. The liquid holding tank of claim 1, wherein the stringer is attached to the first and second walls by at least one of: welding, an adhesive and a mechanical fastener.
 4. The liquid holding tank of claim 2, wherein the first stringer and sidewall are formed together and are seamless.
 5. The liquid holding tank of claim 3, wherein the first wall and the second wall have smooth outside surfaces.
 6. The liquid holding tank of claim 4, wherein the first stringer is hollow.
 7. The liquid holding tank of claim 5, further comprising a second stringer, the second stringer being similar to, and vertically separated from the first stringer.
 8. The liquid holding tank of claim 5, further comprising a second stringer, the second stringer being similar to, and horizontally separated from the first stringer.
 9. The liquid holding tank of claim 6, wherein the first and second walls have a geometric center and wherein the first and second stringers extend between the first and second walls at first and second stringer locations, the first and second stringer locations being equally spaced away from the geometric center.
 10. The liquid holding tank of claim 7, wherein the first and second walls have a geometric center and wherein the first and second stringers extend between the first and second walls at first and second stringer locations, the first and second stringer locations being equally spaced away from the geometric center.
 11. The liquid holding tank of claim 1, wherein the first stringer is comprised of first and second portions, each portion having an annulus shaped face, the first and second portions being joined to each other by the joining of the annulus-shaped faces.
 12. A liquid holding tank comprising: a first sidewall; a second sidewall, the first sidewall and the second sidewall having a first sidewall thickness; a first stringer formed integrally with the first and second sidewalls and extending between the first and second sidewalls inside the liquid holding tank, the stringer being a hollow tube having a tube sidewall, the tube sidewall having a thickness substantially equal to the first thickness.
 13. The liquid holding tank of claim 11, wherein the first and second sidewalls and the stringer are molded at the same time and wherein the hollow tube has a draft of between about two degrees and twenty degrees.
 14. The liquid holding tank of claim 11, wherein the first and second sidewalls have corresponding centroids and wherein the stringer is located at the centroids of the first and second sidewalls.
 15. The liquid holding tank of claim 9, wherein the stringer is configured to exchange heat between air inside the stringer and at least one of liquid and air inside the holding tank.
 16. The liquid holding tank of claim 9, wherein the first portion of a wall stiffener and the first holding tank wall are a unitary structure.
 17. An assembly of reinforced liquid holding tanks, the assembly comprising: first and second reinforced liquid holding tanks, each tank having first and second opposing side walls, the first and second liquid holding tanks being adjacent to each other such that the second side wall of the first liquid holding tank is adjacent to, the first side wall of the second tank; each reinforced liquid holding tank being comprised of: a first holding tank wall; a second holding tank wall substantially parallel to the first holding tank wall and laterally separated from the first holding tank wall by a first separation distance; and a wall stiffener extending between and attached to both the first and second substantially vertical holding tank walls, the wall stiffener being configured to maintain the tank width separation distance, at least where the wall stiffener is attached to the first and second holding tank walls.
 18. The assembly of reinforced liquid holding tanks of claim 16, further comprising a third reinforced liquid holding tank having first and second opposing side walls, such that the first wall of the third tank is adjacent to and in contact with the second wall of the second tank.
 19. The assembly of reinforced liquid holding tanks of claim 16, wherein the wall stiffener in each tank is hollow and the stiffeners in each tank are aligned.
 20. A liquid dispenser comprising: a refrigerated cabinet having an interior width defined by a separation distance between first and second cabinet walls; and first and second reinforced liquid holding tanks located between the first and second walls, each tank having first and second opposing side walls, the first and second liquid holding tanks being configured to be placed adjacent to each other and inside the refrigerated cabinet such that the second side wall of the first liquid holding tank is adjacent to, and in contact with, the first side wall of the second tank; each reinforced liquid holding tank being comprised of: a first holding tank wall having a smooth exterior surface; a second holding tank wall, which is configured to be substantially parallel to the first holding tank wall and, laterally separated from the first holding tank wall by a first separation distance and which also has a smooth exterior surface; and a hollow wall stiffening tube extending integrally formed with the first and second holding tank walls and extending between them, the hollow wall stiffening tube being configured to maintain a separation distance of the walls, at least where the wall stiffener is attached to the first and second holding tank walls, at least when a liquid is inside the holding tank.
 21. The liquid dispenser of claim 20, wherein a first hollow wall stiffening tube in the first tank is aligned with a second hollow wall stiffening tube in the second tank, the aligned wall stiffening tubes configured to allow air to flow between the first and second cabinet walls and configured to enable the first and second liquid holding tanks to move vertically relative to each other when the tanks are holding liquid. 